Optical sensors are widely used in applications ranging from power measurement to imaging. While the requirements for optical sensors vary significantly depending on the requirements of the application, a light sensor suited for imaging must simultaneously provide high responsivity combined with rapid temporal response (typically less than 1/15 of a second).
Digital imaging chips based on silicon photodiodes are the mainstay of current imaging products. Such devices are typically limited to at most one photoelectron per photon and thereby necessitate the use of an extremely low-noise read-out scheme. Furthermore, many digital imaging devices suffer from poor detectivity in the ultraviolet spectrum, particularly in the UV-B and UV-C spectral ranges.
Recently, a new class of solution-processed materials has been shown to be a promising platform technology for photodetectors. However, reports of ultraviolet sensing elements based on solution-processed materials have, however, provided either promising sensitivity (61 A/W) but hundreds-of-seconds temporal response (28); or else fast response but few-mA/W UV responsivity (29).
One potential avenue for improving the performance of photoconductive devices, particular for photon energies lying well above the semiconductor's bandgap, involves the excitation and decay dynamics of multiple carriers. One such effect is multi-exciton generation (MEG), which involves the creation of two or more electron-hole pairs in a semiconductor per absorbed photon (1).
MEG has been recently reported in a wide range of semiconductor materials and structures. Colloidal quantum dot materials in which MEG has been reported experimentally include PbS and PbSe (2), PbTe (3), CdSe (4), and Si (5). In bulk semiconductors, carrier multiplication has been observed repeatedly over the past five decades, both in elemental semiconductors such as germanium (6) and silicon (7), and also in lead chalcogenides (8) including the infrared-bandgap bulk semiconductor PbS (9). Very recently, experiments that carefully account for processes such as photoionization of nanoparticles during spectroscopic studies have evidenced the production of more than one exciton per photon (10) in colloidal quantum dots, with yields ranging from 1.1 to 2.4 excitons per photon (10) when the photon energy exceeds the MEG threshold near ˜Ephoton/Ebandgap>2.7 (11) where Ephoton is the photon energy and Ebandgap the quantum-confined bandgap.
MEG has been reported, based on all-optical spectroscopic data, not only in solution but also in thin solid films; however, in spite of numerous attempts using materials systems and photon energies reported to manifest MEG, neither the external nor the internal quantum efficiency of the photocurrent in a device has been shown to exceed 100% (12-20). In particular, one careful and systematic study (21) recently aimed to explore whether a key signature of MEG—an internal quantum efficiency of greater than unity—was observable in the photocurrent of a low-bandgap PbSe colloidal quantum dot photovoltaic device. Once reflection and absorption were carefully taken into account, IQEs approaching, but not exceeding, 100% were reported.
As a result of these findings, the utility of MEG in optical devices, particular photovoltaic devices, is now regarded as being uncertain. What is therefore needed are devices that utilize MEG to achieve improved photodetector performance.